Antibodies regulate immune responses, not only by recognition of foreign agents but also through the modulation of effector cell function. This latte action is mediated through binding of the antibody constant region, the Fc region, to cell surface receptors. Fc receptors (FcRs), which are expressed by diverse populations of immune cells, can be grouped based on the antibody class they bind. The receptors that bind IgG antibodies, the Fc? receptors, represent the largest family and are of particular interest because antibody binding can either stimulate or attenuate the immune response, depending on the receptor activated. Understanding the function of these receptors and the variation between individuals in their structure and expression is important, not only because of their ability to regulate immune responses, but also because of the increased therapeutic use of therapeutic antibodies, especially in the treatment of cancer. Species differences in the mouse and human have hampered the use of the mouse in translational studies examining the engagement of these receptors by therapeutic human antibodies. In addition, these differences have limited the ability to examine the impact of polymorphisms in these receptors on the pathogenesis of immune diseases and on the responsiveness of patients to therapeutic antibodies. To address this problem we propose to generate a mouse line which expresses the low affinity family of human FCGR genes in place of the endogenous mouse genes. These lines will be easy to breed and thus will be appropriate not only for basic research studies, but also for preclinical evaluation o new therapeutic antibodies.